Over the past several weeks, I have had two NVIDIA cards using the BT869 chipset for TV-Out.
And with both I noticed a severe bug. If you didn't set the refresh rate of the 800x600 mode
to a low refresh-rate. The TV Out mode would not appear in 800x600, even when specifically selecting
800x600. It would instead default to 640x480. This is especially bad with PAL output.

So if you have this particular situation, try lowering the refresh rates.